The present invention relates to using three-dimensional (3D) or multilayer weaving to fabricate fiber reinforcing structures for composite material parts.
More particularly, the invention relates to fiber structures that are to form reinforcement in a composite material part that is a body of revolution presenting, in radial section, a profile that varies (varying in shape and/or in thickness), such as for example an aeroengine fan casing.
Fiber structures that are to constitute the fiber reinforcement of a composite material part, e.g. such as an aeroengine fan casing, are made by 3D or multilayer weaving in a Jacquard type loom, the weaving consisting in inserting weft yarns to create a pattern between warp yarns. The warp yarns are organized in the harness of the loom as a plurality of layers and columns that are manipulated by the loom so as to enable the weft yarns to be inserted in compliance with the weaving pattern(s) programmed in the loom. The weft yarns are inserted between the warp yarns in columns.
In order to enable each column of weft yarns to the inserted while weaving the fiber structure, one or more take-up rollers are used between the outlet of the loom and a storage drum or mandrel onto which the woven fiber texture is wound for subsequent use in forming a fiber preform on an injection mold. The role of this or these take-up rollers located downstream from the loom is to take up the proper lengths of warp yarns after each weft column has been inserted. These rollers have outer surfaces that are covered in a material to which the weaving yarns adhere so as to exert a traction force on the warp yarns.
The fiber texture as woven in this way is to be wound onto an injection mold that is formed by a mandrel having a winding surface presenting, in radial section, a profile in relief corresponding to the profile of the part that is to be fabricated. In order to make a fiber texture that is adapted to the varying shape of the injection mold, use is made of a “contour” weaving technique, which consists in taking up different lengths of warp yarns depending on their positions across the width of the fiber texture that is being woven in the form of a strip. For this purpose, the take-up roller(s) present across their axial width a radius that varies so as to define an outer surface having a profile in relief for taking up different lengths of warp yarn as a function of the positions of the yarns across the width of the fiber texture, a greater length of warp yarn being taken up by the portion(s) of a take-up roller that is/are of radius greater than the remainder of the roller. Differential take-up of the warp yarns depends on the sums of the lengths of fiber texture that are in contact with the take-up rollers. The differential take-up performed by the take-up rollers has the effect, in the weaving cell of the loom, of causing the proper lengths of warp yarns to be pulled prior to inserting the next column of weft yarns.
However, while the fiber texture is being shaped on a mold prior to injection, it is wound to build up a plurality of layers. On each winding turn, the ratios between the various radii defining the profile in relief changes as a function of the number of layers of fiber texture that have already been wound. The magnitudes of these changes in profile increase with increasing shape and/or thickness ratios in the final part. Unfortunately, the above-described fabrication method is capable only of weaving a fiber texture with a profile in relief that is constant, i.e. matching radii for which the cross-section ratios do not change. While the fiber texture is being wound onto the injection mold, it comes out of register because of the differences between the profile as woven, which is constant over the entire length of the woven texture, and the real profile onto which the texture is being wound, thereby giving rise to losses of tension in certain axial positions in the texture. Such unbalances in tension across the width of the part can give rise to numerous defects such as waves, fibers buckling, pinched fibers, zones of unwanted extra thickness, and fiber volume contents that are out of specification. Such unbalances in tension also make the shaping of the fiber texture by winding more complicated, in particular by causing creases or misalignments to be formed, thereby making the shaping of the fiber texture more arduous and time-consuming.